Load leveler for pulse modulator



March 21, 1967 R. H. MILLER 3,310,684

LOAD LEVELER FOR PULSE MODULATOR Filed April 29, 1963 LINE TYPE GRID MODULATOR PULSER LINE TYPE GRID MODULATOR PULSER INVENTOR ROGER H MILLER BY M4 W A 7' TOPNE Y United States Patent ice 3,310,684 LOAD LEVELER FOR PULSE MODULATOR Roger H. Miiler, Mountain View, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Fiied Apr. 29, 1963, Ser. No. 276,670 3 (Itaims. (Cl. 307-39) The invention disclosed herein was made under, or in, the course of Contract No. AT(04-3)-363 with the United States Atomic Energy Commission.

The present invention relates generally to voltage stabilizing circuits and in particular to a load-leveler circuit employed in conjunction with a line-type modulator which pulses a grid-controlled electron gun for circumventing the formation of voltage transients in the electron gun.

One of various conventional solutions to the problem of voltage transients appearing in a grid-controlled electron gun is to design the gun cathode pulser, or modulator, with a much lower impedance than the electron gun and to terminate the pulser or modulator, pulse-forming network with a resistive load which draws much more current than the electron gun. In general, the ratio of the voltage change produced by the gun current to the cathode voltage is equal to the ratio of the gun current to the total load current. Thus, if at least one percent cathode voltage stability is required, the resistive load must dissipate 99% of the power delivered by the modulator while only 1% is delivered to generate a useful electron beam. The inefiiciency of such a load-leveler system is obvious,

The present invention overcomes the above shortcomings by providing a triode, or other similar,- suitable valve or switching means, in parallel with part or all of a secondary, resistive load of an electron gun, wherein the secondary resistive load is of relatively insignificant value in comparison to the resistive load of the previously noted prior art circuit. That is, the load-leveler circuit set forth in accordance to the mechanism of the invention prevents gun voltage transients from occurring in the gun circuit by providing an alternate path for gun current flow, thereby to maintain the current load constant, as seen by the line-type modulator or similar cathode pulser means, without the necessity of dissipating large amounts of power.

Accordingly, it is an object of the present invention to provide a circuit for improving the efficiency of a linetype modulator utilized in conjunction with a modulated load system.

Another object of the present invention is to provide a iload-leveler circuit capable of circumventing the generation' of voltage transients in a modulated load, such as a grid or anode controlled electron gun or microwave tube in a simple and inexpensive manner.

Still another object of the present invention is to provide a load-level-er circuit capable of eliminating gun voltage transients in a grid controlled electron gun by maintaing the current iload on the electron guns cathode pulser means constant.

It is still a further object of the present invention to provide an alternate current path in parallel with part or all of the resistive load of a grid controlled, electron gun.

Yet another object of the present invention is to provide a load-leveler circuit for use with an electron gun, which circuit utilizes a triode wherein only a portion of the relatively high voltage appearing on the electron gun cathode is held off by the triode.

Another object of the present invention is to provide a load-leveler circuit for use with a grid controlled electron gun which circuit draws current from the cathode pulser means when the gun is cut ofi? but not when the gun is firing.

I, rattan Patented Mar. 21, 1967 Additional objects and advantages of the invention will become apparent from the following description and claims considered together with the accompanying drawing, of which FIGURE 1 is a schematic diagram illustrating the loadleveler circuit of the present invention as utilized, for example, in conjunction with a grid controlled, electron gun, and

FIGURE 2 is an alternative embodiment of the loadleveler circuit of FIGURE 1.

Referring to FIGURE 1, there is shown a load-leveler circuit 10 employing the mechanism of the present invention as exemplified in use with a conventional grid controlled electron gun 12 having a cathode 11 and an anode 13, an electron gun grid pulser means 14,. and an electron gun cathode pulser means such as, for example, a line-type modulator 16. It is to be understood that the invention is not particularly limited to use with the electron gun circuit of previous mention, but can be adapted for use with other modulated load systems, wherein it is desirous to feed a pulse to a modulated load with a minimum generation of load voltage transients. For example, the invention concept can be utilized to prevent voltage transients in radar circuitry wherein a pulser means drives a microwave tube having a modulated grid or anode.

The design of the load-leveler circuit 10 utilizes a triode tube 18 having a plate 19, a grid 20, and a cathode 21. The cathode 21 and one end of a resistor 22 are connected to a negative output terminal of modulator 16. The other end of resistor 22 is connected to a load resistor 23 to thereby form of the two resistors 22, 2 3 a load resistive means. The common junction of resistors 22 and 23 is connected to the plate 19 of the triode tube 18, whereby the two resistors 22, 23 operate, in essence, as a voltage divider means which supplies the proper plate operating voltage to the triode tube 18. A variable resistor 24 is connected at one end to the cathode 21 of triode 18 and at its other end to a resistor 26, wherein such variable resistor 24 provides the proper grid bias to the grid 20. The opposite end of resistor 26 is connected to the free end of resistor 23, such opposite end of resistor 26 and free end of resistor 23 being in fact connected to a positive terminal of the modulator 16.

A secondary coil of an isolation transformer 28 is connected from the common junction of variable resistor 24 and resistor 26, to the grid 29 of triode 18. A primary coil of the isolation transformer 28 is connected across the output terminals of the grid pulser means 14. The primary and secondary coils of the isolation transformers 28, 28, are connected so that the pulse applied to the grid 20 of the triode 18 is opposite to the pulse from the pulser 14, as shown by the polarity dots in the figure. The out-put terminals of grid pulser means 14 are also connected across the cathode and grid of the electron gun 12. The cathode of electron gun 12 is further connected to the negative output terminal of modulator 16, while the anode 13 is connected to the positive output terminal of modulator 16.

In operation, the modulator 16 is pulsed to periodicalily apply a negative voltage to the cathode of gun 12, thereby providing for the pulsed generation of electron emission therefrom. However, no electrons flow from the cathode-since the grid of the gun 12 is biased negatively. At such time as the electron gun 12 is in this non-conducting state, the resistor 24 is adjusted to bias the grid 20 sufliciently positive to cause the triode 18 to conduct. Thus, when the electron gun is biased off, the circuit provides a bypass path for that portion of the current which generally flows to the electron gun at such time as the gun is in the conducting state.

When electron flow in the gun is desired, grid pulser means 14 is triggered to generate a positive pulse which is delivered to the gun grid to make the latter positive with respect to the cathode of the gun 12. The positive pulse from the grid pulser means 14 is simultaneously introduced to the primary coil of the isolation transformer 28, wherein it is inverted to a negative pulse by the polarity relationship of the secondary and primary coils. The secondary coil applies the negative pulse to the grid 20 of triode 18 to thereby drive it sufiiciently negative to cut off triode 22. Therefore the same pulse which triggers the electron gun 12 is, in essence, used to cut 011 the triode. Thus the current flowing through the triode 22 is redirected to the electron gun 12 without the buildup of gun voltage transients. That is, the current load (i.e., impedance) as seen by the modulator 16 remains constant even though the electron gun 12 has been triggered. It may be seen that the triode 18 draws current from the cathode pulse forming modulator 16 only when the gun 12 is cut oif and not when the gun is firing. To reiterate, by providing this alternate path for current flow at such time as the gun 12 is in a non-conducting state, the current load as seen by the modulator 16 remains constant when the gun is triggered, thereby circumventing the generation of gun voltage transients.

As may be seen from the circuit the triode tube 18 does not have to stand oil? the full electron gun voltage, but only the maximum plate voltage. This maximum plate voltage is equal to the product of the gun current times the gun voltage, divided by the maximum plate current. That Gun Current X Gun Voltage Max. Plate Current Further, the ratio of gun current to current delivered by the cathode pulser means 16 is not greater than the ratio of the triode plate voltage, when the gun is cut 011, to the gun voltage. For example, if the gun draws 2 amperes and is pulsed to 100 kilovolts and the triode tube used has a plate-rated voltage of 25 kilovolts, then the modulator must deliver a peak current of a little more than 8 amperes. For the above case, the triode tube must be biased to draw 8 amperes.

FIGURE 2 shows a schematic diagram of an alternative embodiment of the present invention as utilized with the electron gun 12, the grid pulser means 14, and the linetype modulator 16. The negative terminal of modulator 16 is connected to one end of a resistor 32, to a cathode 34 of a triode tube 18, to one side of a primary coil of an isolation transformer 28, and to the cathode of the electron gun 12. A resistor 36 and resistor 38 are serially connected to the other end of resistor 32; wherein the resistors 32, 36 and 38 comprise in essence a secondary, resistive load means. The common junction between resistors 32 and 36 is connected to a grid 40 of the triode tube 18; the resistors 32 and 36 acting as a voltage divider means, whereby positive grid bias is provided to tube 18 at such time as the gun 12 is in a non-conducting state. The common junction between resistors 36 and 38 is connected to a plate 42 of the triode tube 18', and supplies the necessary plate voltage thereto. A secondary coil of the isolation transformer 28 is connected at one end to the cathode 34 of the triode tube 18, and at the other end thereof to the grid 40 of the triode by way of a coupling capacitor 44. The grid of the electron gun 12 is connected to the end of the primary coil of isolation transformer 28 opposite that end thereof connected to the negative terminal of the modulator 16. The output terminals of the grid pulser 14 are connected across the primary coil of isolation transformer 28, and also across the cathode and grid of the electron gun 12.

The operation of the alternative embodiment of the invention shown in FIGURE 2 is, in essence, identical to the operation of the circuit of FIGURE 1, except for the mode of biasing the grid 40 of the triode tube 18'. That Max. Plate Voltage:

is, as shown in FIGURE 2, the grid 40 is biased by means of the resistor 32, which is in series with the load resistors 36 and 38 of the invention. On the other hand, in the embodiment of FIGURE 1, the grid 20 of triode tube 18 is biased by means of the resistor 24 in the separate series of resistors 24 and 26. More particularly, referring to FIGURE 2, at such time as the electron gun 12 is in the non-conducting state, the triode tube 18' is in the conducting state, and a current path is provided through the tube and through the portion of the resistive load consisting of resistor 38. At such time as the grid pulser means is energized to introduce a positive pulse to the grid of the electron gun 12, isolation transformer 28' is disposed to receive and invert the same positive pulse, and apply the resultant negative pulse to the grid 4-0 of the triode 18 to drive the grid negative and thus the triode to a non-conducting state. Thus the current flow normally passing through triode 18' is rerouted to electron gun 12, with a minimum generation of voltage transients. The modulator 16 sees no variation in the current load when the electron gun 12 is triggered.

There are other variations of circuit design possible as regards the inventive concept herein utilized, other than the use of a different mode of triode grid biasing. For example, the isolation transformer (28, 28') utilized in the embodiments of FIGURES 1 and 2 to invert the positive pulse of the grid pulser means and apply the resulting negative pulse to the triode tube grid to drive same negative, may be replaced by a dropping resistor and amplifying means. More particularly, the dropping resistor is serially disposed in the electron gun cathode input line, to provide a signal proportional to the electron gun current across the resistor, which signal is introduced to the amplifying means to be amplified and fed to the grid of the triode tube. As such time as the electron gun is in the non-conducting state, the amplifying means must provide a positive bias to the grid of the triode tube to allow conduction therethrough. As such time as the electron gun is triggered by a positive pulse from the grid pulser means, the change of signal extracted from across the dropping resistor is fed to the amplifying means, and the resulting amplified signal is introduced to the grid of the triode tube to drive the latter into a non-conducting state. Thus, the current path is redirected from the triode tube to the electron gun with no appreciable variation of current load as seen by the gun cathode modulator. The use of a dropping resistor and amplifying means in place of the isolation transformer provides for operating the grid pulser means over a predetermined range without losing the load-leveler characteristics set forth in the invention. That is, by utilizing an amplifying means which amplifies a signal proportional to the electron gun beam current, and op erating it over its linear range, the output pulse of the grid pulser means can be varied over a region, and the load-leveler circuit of the invention will continue to stabilize the current load. This is possible since the varying grid pulse applied to the gun will give rise to a varying signal across the dropping resistor which is proportional to the beam current. The varying signal is then proportionally amplified and applied to the triode tube by the linearly operated amplifying means.

As an example of specific parameters which may be employed in the circuit of the present invention, the following are parameters included by way of example with the circuit of FIG. 1.

Resistor 22:7.5K ohms Resistor 23:12.5K ohms Variable resistor 24 is adjusted to -1.8K ohms Resistor 26=10OK ohms Triode tube 18 is a 6021 having a thoriated-tungsten filament and a maximum plate voltage of 30 kv.

The line-type modulator 16 is generally of the type which utilizes a pulse-forming network and .a gas-filled switch tube, a d which in the present application has a pulsed output of -80 kv. as applied to the cathode of the electron gun 12. The grid pulser means used to apply the positive pulse to the grid of the electron gun is generally of the hard-tube variety. Examples of the line'type modulator, the grid pulser means and the electron gun may be found in the MIT Radiation Laboratory Series, Mc-

Graw-Hill Book Co., Inc., vol. 3, Chap 12; vol. 5, Chap. 1,

2; and vol. 22, Chap. 2, respectively.

While the invention has been disclosed with respect to several specific embodiments, it will be apparent to those skilled in the art that numerous variations and modifications may be made Within the spirit and scope of the invention, and thus it is not intended to limit the invention except as defined in the following claims.

What is claimed. is:

1. In a line-type modulating system, the combination of:

(a) a tube means loading device having first and second current conducting electrodes for conducting up to a full load current and a control electrode for control of the load current therebetween, said device being normally maintained in a nonconducting state;

(b) a line-type modulator having first and second out put terminals connected respectively to the first and second current conducting electrodes of said tube means for application of a modulator pulse thereto;

(c) a load leveler connected between the output terminals of said line-type modulator, said load leveler being arranged to conduct a current at least equal to the full load current of said loading device upon application of a modulator pulse thereacross, said load leveler including a control input;

(d) a pulser having first and second output terminals connected respectively to the first electrode and control electrode of said tube means loading device for application of a control pulse thereto, which control pulse is timed to occur within the duration time of the modulator pulse, said loading device being triggered thereby to conduct a load current; and

(e) means connecting the output terminals of said pulser to the input means of said load leveler tor application of the control pulse thereto simultaneously with application of the control pulse to said loading device, said load leveler being responsive to the control pulse to conduct less than full load current by an amount substantially equal to the load current in said loading device, so that the voltage across said loading device is maintained constant.

2. A line-type modulating system according to claim 1, wherein said load leveler comprises:

(a) a pair of resistors connected in series across the output of said modulator;

(b) switch means connected in parallel across one of said resistors;

' (e) bias means for controlling said switch means to conduct current upon occurrence of the modulator pulse thereacross, the amount of current through said switch means being substantially equal to at least the full load current of said loading device; and

(d) means coupling said control input to said switch means for applying the control pulse thereto from said pulser, said control pulse being of a polarity and magnitude to cause said switch means to decrease current conduction by an amount equal to the increase of current through said loading device.

3. A line-type modulating system according to claim 2 wherein said switch means is an electron tube having a grid for control thereof, said loading device is an electron gun having a grid for control thereof, and the control pulse from said pulser is applied with a positive polarity to the grid of said gun and with a negative polarity to the grid of said tube.

References Cited by the Examiner UNITED STATES PATENTS 2,302,900 11/1942 Vance 323-4 X 2,404,624 7/ 1946 Dome 328267 X 2,575,232 11/1951 Parker 315-l27 X ORIS L. RADER, Primary Examiner.

T. J. MADDEN, Assistant Examiner. 

1. IN A LINE-TYPE MODULATING SYSTEM, THE COMBINATION OF: (A) A TUBE MEANS LOADING DEVICE HAVING FIRST AND SECOND CURRENT CONDUCTING ELECTRODES FOR CONDUCTING UP TO A FULL LOAD CURRENT AND A CONTROL ELECTRODE FOR CONTROL OF THE LOAD CURRENT THEREBETWEEN, SAID DEVICE BEING NORMALLY MAINTAINED IN A NONCONDUCTING STATE; (B) A LINE-TYPE MODULATOR HAVING FIRST AND SECOND OUTPUT TERMINALS CONNECTED RESPECTIVELY TO THE FIRST AND SECOND CURRENT CONDUCTING ELECTRODES OF SAID TUBE MEANS FOR APPLICATION OF A MODULATOR PULSE THERETO; (C) A LOAD LEVELER CONNECTED BETWEEN THE OUTPUT TERMINALS OF SAID LINE-TYPE MODULATOR, SAID LOAD LEVELER BEING ARRANGED TO CONDUCT A CURRENT AT LEAST EQUAL TO THE FULL LOAD CURRENT OF SAID LOADING DEVICE UPON APPLICATION OF A MODULATOR PULSE THEREACROSS, SAID LOAD LEVELER INCLUDING A CONTROL INPUT; (D) A PULSER HAVING FIRST AND SECOND OUTPUT TERMINALS CONNECTED RESPECTIVELY TO THE FIRST ELECTRODE AND CONTROL ELECTRODE OF SAID TUBE MEANS LOADING DEVICE FOR APPLICATION OF A CONTROL PULSE THERETO, WHICH CONTROL PULSE IS TIMED TO OCCUR WITHIN THE DURATION TIME OF THE MODULATOR PULSE, SAID LOADING DEVICE BEING TRIGGERED THEREBY TO CONDUCT A LOAD CURRENT; AND (E) MEANS CONNECTING THE OUTPUT TERMINALS OF SAID PULSER TO THE INPUT MEANS OF SAID LOAD LEVELER FOR APPLICATION OF THE CONTROL PULSE THERETO SIMULTANEOUSLY WITH APPLICATION OF THE CONTROL PULSE TO SAID LOADING DEVICE, SAID LOAD LEVELER BEING RESPONSIVE TO THE CONTROL PULSE TO CONDUCT LESS THAN FULL LOAD CURRENT BY AN AMOUNT SUBSTANTIALLY EQUAL TO THE LOAD CURRENT IN SAID LOADING DEVICE, SO THAT THE VOLTAGE ACROSS SAID LOADING DEVICE IS MAINTAINED CONSTANT. 